DOC: Edit README notes on unit/integration tests
DOC: Edit README notes on unit/integration tests

to get them out of the way a bit (so that they don't look like the
(confusing) default thing you should do when trying out Uptane).
Also explain why they may be useful.

shikhar394/uptane,shikhar394/uptane,uptane/uptane,awwad/uptane,awwad/uptane,uptane/uptane